EntityRelationship

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Entity-Relationship Methods in DBMS
Database Management Prof.. Boehrer Group Project
2 Wednesday October 13th 1115 - 1210
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Project Team
Darren A. Blair
Kim Frank
Phillip Limitone
Susan Varghese
Leigh Weston
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What is an Entity Relationship?
An Entity is a person, place, event or concept,
which represents the structure or data
organization. Each entity is stands alone and
occurs once. The entities may share common
properties. Entities are graphically represented
in capital letters and in a box.
Relationships are the associations amongst the
entities that is of interest to the organization.
Relationships are also referred to as the glue
that holds the entities together (McFadden,
Hoffer 1994).
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Entities

• Entities

Employee Henry Johnson Customer 123 Order
0234
EX

• Class

EX
CUSTOMER CustomerNumber CustomerName Address City
State Zip ContactName PhoneNumber
EMPLOYEE EmployeeNumber EmployeeName Address City
State Zip SocialSecurity PhoneNumber
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• Instance

EX
Instance of the Entity EMPLOYEE 056 Mary
Smith 56-63 74th Street Glendale NY 11385 070-50-1
265 718-897-2705
CUSTOMER 12356 Henrys Hardware 510 Robinson
Lane Patterson NJ 10452 Gerry Jr. 914-965-2301

• Attribute

EX
EmployeeName CustomerNumber ContactName
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• Identifiers

CUSTOMER instances could be identified by
CustomerNumber or CustomerName. Employee
instances could be identified by
EmployeeNumber, EmployeeName or SocialSecurity
because no two customers can have the same
number, and no two people can have the same
social security number. A Non-unique identifier
would be EmployeeName because there could be two
employees with the same name.
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EMPLOYEE entity contains EmployeeNumber EmployeeNa
me Address City State Zip Social Security
Number Phone Number Instance of
Employee 056 Mary Smith 56-03 74th
Street Glendale NY 11385 070-50-1265 718-897-2705
Entity Class--gt
Attributes--gt and Identifiers
Instance--gt
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Types of relationships
1 to 1
Truck
1 1
Engineer
1 to N
Engineer
1 N
Service
N to M
Service
N M
Client
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ER Diagrams
-Ellipse
-Rectangle
Represent entity sets
Represent attributes

-Line
-Diamond
Link attributes to entity sets and entity sets
to relationships
Represent relationship among entity sets
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Weak Entity
A weak entity is an entity that cannot exist in
the database unless another type of entity exists
also.
Strong Entity
A strong entity are all other entities.
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Employee
Truck-Assgn

Engineer
Eng-Skill
Truck
1 1
1 N
I N
Service Provider
Engineer Certification
Service
N I
N M
Qual-Engineer
Fee
Client Service
Certification
Client
1 N
Referred-By
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For more information, you can visit the following
links.
http//www.islandnet.com http//www1.cern.ch/Adamo
/guide/Chapter-2.html http//scis.nova.edu/bernst
r/weekly04.html http//www.lis.pitt.edu/sochats/I
S2071/ermodel.html
This file is also located in Prof. Boehrer's Out
directory